Measurement of the photon electroproduction cross section at JLab with the goal of performing a Rosenbluth separation of the DVCS contribution

Resumen

An elementary particle is a particle whose substructure is unknown, thus it is unknown whether it is composed of other particles. Historically, the atom constituent particles (electrons, protons and neutrons) were all regarded as elementary particles. However, with the discovery of the magnetic moment of the proton, it became clear that the proton did not belong to this category. With further experimentation, more evidence was found that protons and neutrons, as well as all hadrons, were composed of other particles, and had an internal structure. The first evidence for quarks as real constituent elements of hadrons was obtained in late 1967, when the first of a long series of experiments on highly inelastic electron scattering was started at the two mile accelerator at the Stanford Linear Accelerator Center (SLAC). The raw counting rates were much higher than it had been expected in the deep inelastic region, where the electron imparts most of its energy to the proton. The experiment showed that the probability of deep inelastic scattering decreased much more slowly with the invariant momentum transfer to the proton, than that for elastic scattering. A way to interpret this unexpected behavior was that the electrons were hitting some kind of hard core inside the target protons. The first evidence for gluons came in three jet events at PETRA. In the summer of 1973 physicists at Harvard and Princeton demonstrated that in certain gauge theories the force between the quarks could become relatively weak at short distances, a behavior known as asymptotic freedom, which causes bonds between particles to become asymptotically weaker as energy increases and distance decreases. Another property of the interactions between quarks, known as confinement, states that the force between quarks does not diminish as they are separated. Because of this, when two quarks become separated, as happens in particle accelerator collisions, at some point it is more energetically favorable for a new quark-antiquark pair to spontaneously appear, than to allow the distance to extend further. Although analytically unproven, confinement is widely believed to be true because it explains the consistent failure of free quark searches. There is no known phase-transition line separating these two properties. Confinement is dominant in low-energy scales but, as energy increases, asymptotic freedom becomes dominant. These interactions between partons, quarks and gluons, are described by the quantum chromodynamics (QCD) theory. Nowadays, Deep Inelastic Scattering (DIS) continues to offer a path to extract new information on the hadronic structure, and consequently, on the unknowns of how QCD works. The experiment this thesis is based on, Jefferson Lab E07-007, seeks to exploit this kind of process, and more specifically, the Virtual Compton Scattering (VCS) process, which is an exclusive reaction of DIS. VCS consists in the production of a real photon off the nucleon, when scattered by a virtual photon. This kind of reactions, in a certain kinematic regime known as Deeply Virtual Compton Scattering (DVCS), can provide interesting information about a new class of quark and gluon matrix elements, called Generalized Parton Distributions (GPDs). GPDs correlate the transverse spatial distribution of the struck parton (quark or gluon) with the light-cone momentum fraction of the parton in the target. This provides us with information about spatial and momentum distributions of partons within the nucleon, which helps us understand how the behavior of partons conferes the characteristics of the nucleon. Due to the small cross section of DVCS, in order to conduct these kind of experiments it is necessary to make use of facilities capable of providing high beam intensities. One of these facilities is the Thomas Jefferson National Accelerator Facility, where the experiment JLab E07-007, ``Complete Separation of Virtual Photon and pi0 Electroproduction Observables of Unpolarized Protons", took place during the months of October to December of 2010. I started my collaboration with the experiment several months after the data acquisition. Here I present my work on the data analysis as well as the computed cross sections of the studied reaction. Chapter 1 is a theoretical introduction to the study of the nucleon structure, reviewing the concepts of form factors and parton distributions through elastic and inelastic processes. The computation of the photon leptoproduction cross section is described in detail, as well as the goals of experiment E07-007. Chapter 2 is a description of Jefferson Lab main characteristics, focusing on the experimental Hall A, where the experiment took place, and its instrumentation. The experimental setup along with the kinematics employed during data acquisition can be found in this chapter. Special detail is given to the electromagnetic calorimeter, the device on which most of the work of this thesis is based on. Chapter 3 describes the analysis of the data stored by the electromagnetic calorimeter, with the purpose of obtaining the kinematic variables of the real photons resulting from DVCS reactions. This chapter also includes the process of calibration of this apparatus as well as the computation of its energy and angular resolution. Chapter 4 describes the selection of events from stored data, the applied cuts to kinematical variables and the background subtraction. Also, the process of extraction of the necessary observables for computing the photon leptoproduction cross section is described, along with the main steps followed to perform the Monte Carlo simulation used in this computation. The resulting cross sections are shown at the end of the chapter.